An ambulance whizzes by, and suddenly its siren drops in pitch. We are all familiar with the Doppler effect, even if we don’t know it by name. Now, scientists have found an alternative version of the phenomenon, for when sound, or light, scatters off a rotating object. The discovery could enable astronomers to measure a distant planet’s rotation, or even improve the performance of wind turbines.
Here’s how the Doppler effect works: When a noisy object is moving toward you, its sound waves bunch up, producing a higher frequency, or pitch. Conversely, as soon as the object is moving away from you, the sound waves stretch out, and the pitch lowers. The faster the object, the greater the pitch change.
The Doppler effect occurs for light as well as sound. For instance, astronomers routinely determine how fast stars and galaxies are moving away from us by measuring the extent to which their light is “stretched” into the lower frequency, red part of the spectrum. Redshifts like this were famously used in the 1920s to infer that most stars and galaxies are moving away from us and that the universe must be expanding.
Redshifts in light, and the pitch-drop of passing sirens, are examples of a linear Doppler effect. In recent decades, however, scientists have discovered that the Doppler effect also exists for objects that are rotating, the so-called orbital angular momentum (OAM) of light, and now researchers they believe they can determine how fast a planet is spinning.
Back on Earth, he adds, lasers that measure the OAM could be sent through wind farms to determine the rotation of air currents. “If you’re a windmill, you’d like to know in advance of blustery wind, so you can tether your blades.”
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Reported by JON CARTWRIGHT of ScienceNOW. This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.orgRead More